(1) Field of the Invention
The present invention generally relates to a color image forming system utilizing an electrophotographic method, such as a color copier, a color printer or a color facsimile, and more particularly to a color image forming system in which toner images related to a plurality of colors are sequentially transferred from a photoconductive medium to an intermediate transfer belt in an overlaying manner, and a multicolor image on the intermediate transfer belt is transferred to a recording material.
(2) Description of the Related Art
In a color image forming system, a photoconductive medium serving as an image support is rotated and the photoconductive medium surface is charged by a charger. A laser scan unit scans the charged surface of the photoconductive medium by a laser beam which is turned ON/OFF depending on a color-separated image signal. The color-separated image signal is generated with respect to each of secondary colors, such as yellow, cyan and magenta. The laser scan unit sequentially outputs respective laser beams related to the secondary colors.
In the laser scan unit, a polygonal mirror is rotated by a polygonal motor, and a laser beam emitted by a laser light source is deflected by the polygonal mirror through the rotation of the polygonal mirror. The laser beam deflected by the polygonal mirror scans the charged surface of the photoconductive medium in a main scan direction. Hence, the photoconductive medium surface is exposed by the laser beam and an electrostatic image related to one of the secondary colors is formed on the photoconductive medium. Each of the electrostatic images related to the secondary colors is visualized by a developing unit by adhering a corresponding one of secondary color toners onto the photoconductive medium.
Such toner images are sequentially transferred from the photoconductive medium to an intermediate transfer belt in an overlaying manner. The intermediate transfer belt serves as an image support and is rotated in accordance with the rotation of the photoconductive medium. Hence, a multicolor image is formed on the intermediate transfer belt, and the multicolor image is transferred from the intermediate transfer belt to a recording material, such as a copy sheet or a plastic sheet.
In the color image forming system, a reference mark is provided at a reference position on the intermediate transfer belt, the reference mark being located outside an image forming area of the intermediate transfer belt. A reference mark sensor is provided to output a reference mark signal every time the reference mark at the reference position on the intermediate transfer belt is detected during rotation of the intermediate transfer belt. The reference mark signal is output by the reference mark sensor at a time the reference position passes through the reference mark sensor per revolution of the intermediate transfer belt.
In the above-mentioned color image forming system, a laser scan start timing for the laser scan unit to start scanning the photoconductive medium by the laser beam is controlled based on the reference mark signal output by the reference mark sensor. According to this control, the scanning of the photoconductive medium in each main scan line is started by the laser scan unit in accordance with the reference mark signal, and the laser scan unit outputs a line sync signal in accordance with the reference mark signal. The scanning of the photoconductive medium in each main scan line by the laser beam is performed in accordance with a period of the line sync signal.
If a peripheral length of the intermediate transfer belt (which is proportional to a time for one revolution of the intermediate transfer belt) is equal to an integral multiple of the period of the line sync signal, no displacement of the toner images on the intermediate transfer belt will be produced when sequentially transferring the toner images from the photoconductive medium to the intermediate transfer belt.
However, it is practically difficult that the peripheral length of the intermediate transfer belt is exactly set at an integral multiple of the period of the line sync signal. Usually, in the above-described color image forming system, the peripheral length of the intermediate transfer belt is deviating from the integral multiple of the period of the line sync signal. The timing for the reference mark sensor to output the reference mark signal may be shifted from the timing for the laser scan unit to output the line sync signal at which the laser scan unit starts scanning the photoconductive medium by the laser beam. This causes displacement of the toner images on the intermediate transfer belt. In such a case, the multicolor image on the intermediate transfer belt has the toner image displacement, and it is difficult for the above-described color image forming system to provide a good-quality color image on the recording material.
One conceivable method to eliminate the above-mentioned problem is to arrange the intermediate transfer belt in the color image forming system, the peripheral length of the intermediate transfer belt being exactly equal to an integral multiple of the period of the line sync signal. However, the intermediate transfer belt is subjected to wavy movement or vibration during the rotation even if the peripheral length of the intermediate transfer belt is accurate. Thus, it is difficult for the above-mentioned method to prevent the timing of outputting the reference mark signal by the reference mark sensor from being shifted from the laser scan start timing by the laser scan unit.